Never dress casual. Wear business casual attire. This means leather shoes, white dress shirt, and slacks (or khakis since, as the above person stated, it is an on campus job). Don't really need a tie for a work study job.
Comb your hair, use gel or wax. Also, bring a leather portfolio. I know Ackerman sells some cheap ones for $15 or so, but I suggest you get one of those $30 ones. Consider it a long term investment as you'll use it in the future. It's always nice to have paper and a pen ready. Plus, print several copies (yes, several) of your resume and put it in there. Your interviewer is likely to ask for a copy of your resume, even though you submitted it online.
Don't use a plastic binder or folder you'd normally use for class. It looks very unprofessional.
He or she will most likely ask you questions like, "Tell me about yourself," "Why do you want to work here?" "What related work experiences have you had prior to this job?"
Also in the end, they will ask you if you have any questions. ALWAYS ask a question or two.
The interview shouldn't take more than 30 minutes really.